The present invention relates to a poloidal coil arrangement for a nuclear fusion apparatus, and more particularly, to an improvement for connecting portions in which a plurality of divided poloidal coils are connected to each other through joining members.
A conventional nuclear fusion apparatus of the torus-type comprises a vacuum vessel, a plurality of toroidal coils, and a plurality of poloidal coils in order to confine and heat a plasma generated within the vacuum vessel. The vacuum vessel is of a hollow doughnut shape, and the poloidal coils extend along the vacuum vessel in the circumferential direction with respect to the longitudinal axis of the apparatus while the toroidal coils extend perpendicularly to the poloidal coils and the vacuum vessel and surround them. The poloidal coils and the vacuum vessel are concentrically disposed.
Accordingly, in order to assemble the apparatus, the plurality of toroidal coils must be divided into at least two sections, or the poloidal coils and the vacuum vessel must be circumferentially divided into at least two sections. In the latter case, the vacuum vessel and the respective poloidal coil sections are separated to form a sufficiently large gap therebetween for allowing the toroidal coils to pass through the gap. The vacuum vessel and the poloidal coil sections are then electrically and mechanically joined by connecting members for bridging the gap using screw or bolt joints for example. An example of such a screw joint is described in detail in an article entitled "The Toroidal and Poloidal Magnetic Field Coils of the ASDEX Tokamak", by R. Allgeyer et al., in "Proceedings of the Sixth Symposium on Engineering Problems of Fusion Research", IEEE Pub. No. 75CH1097-5-NPS, Nov. 20, 1975, pp. 777-780. However, the conventional screw joint which requires a certain conductor thickness is not suitable for use in a recent large, sophisticated nuclear fusion apparatus in which the conductor coil turns are thin and have cross sectional dimension of, e.g., 4 mm.times.125 mm, required to cope with the limited space within the apparatus.
Alternatively, an example of a silver braze joint for coil turn sections is disclosed in an article by M. W. G. Bahr et al. entitled "Silver Brazing Methods for Joining the OH and SF Field Coil Turns for the PLT Machine" in "Proceedings of the Sixth Symposium on Engineering Problems of Fusion Research", IEEE Pub. No. 75CH1097-5-NPS, Nov. 19, 1975, pp. 480-483. The braze joint is more suitable for the recent apparatus than the screw joint because the conductor thickness is not a requisite condition. However, since the braze joint requires the successive heating of the coil turns of the conductor sections, insulating coatings on the conductor turns or the surrounding devices may be damaged by a torch during the brazing operation.
In a further detailed conventional apparatus using braze joints, each braze joint has the cross-sectional shape of an inverted trapezoid, and conductor turns have projecting portions for receiving the slanting sides of the inverted trapezoid of each braze joint. The end surfaces of the slanting sides of each braze joint and the end surfaces of the projecting portions of the conductor turns are heated at a temperature of e.g., 700.degree. to 800.degree. C. to weld each braze joint to each conductor turn. The braze joints of this shape have advantages in that the joints are supported entirely by the conductor turns and the weld length becomes longer. However, in this apparatus, when a joining member is welded to a conductor turn, the operation of a welding torch is hindered by projecting portions of conductor turns located above the conductor turn being welded, and insulating coatings on the conductor turns or other surrounding devices may be damaged during the welding operation, and conductor turns are not fully welded to each other through joining members. To overcome this problem, in another apparatus, the length of each braze joint with respect to the circumferential direction increases as conductors are turned so that the welding operation is not hindered by projecting portions of conductor turns. However, this conventional apparatus also has disadvantages in that it is difficult to assemble the apparatus since a space for the welding operation cannot be assured due to the increasing lengths of the braze joints with respect to the poloidal direction, and that the braze joints have sizes different from each other.